Uit de Feynman Lectures on Physics, Vol. I (Addison-Wesley, 1963)

Poincaré made the following statement of the principle of relativity:
"According to the principle of relativity, the laws of physical
phenomena must be the same for a fixed observer as for an observer who
has a uniform motion of translation relative to him, so that we have not,
nor can we possibly have, any means of discerning whether or not we are
carried along such a motion."

When this idea descended upon the world, it caused a great stir among
philosophers, particularly the "cocktail-party philosophers", who say,
"Oh, it is very simple: Einstein's theory says all is relative! In fact,
a surprisingly large number of philosophers, not only those found at
cocktail parties (but rather than embarrass them, we shall just call them
"cocktail-party philosophers"), will say, "That all is relative is a
consequence of Einstein, and it has profound influences on our ideas."
In addition they say "It has been demonstrated in physics that phenomena
depend upon your frame of reference." We hear that a great deal, but it is
difficult to find out what it means. Probably the frames of reference that
were originally referred to have had a profound effect on modern thought.
One might well wonder why, because, after all, that things depend upon
one's point of view is so simple an idea that it certainly cannot have been
necessary to go to all trouble of the physical relativity theory in order
to discover it. That what one sees depends upon his frame of reference is
certainly known to anybody who walks around, because he sees
an approaching pedestrian first from the front and then from the back;
there is nothing deeper in most of the philosophy which is said to have come
from the theory of relativity than the remark that "A person looks different
from the front than from the back." The old story about the elephant that
several blind men describe in different ways is another example, perhaps,
of the theory of relativity from the philosopher's point of view.

But certainly there must be deeper things in the theory of relativity than
just this simple remark that "A person looks different from the front than
from the back." Of course relativity is deeper than this, because we can
make definite predictions with it. It certainly would be rather remarkable
if we could predict the behavior of nature from such a simple observation
alone.

There is another school of philosophers who feel very uncomfortable about
the theory of relativity, which asserts that we cannot determine our absolute
velocity without looking at something outside, and who would say, "It is
obvious that one cannot measure his velocity without looking outside.
It is self-evident that it is meaningless to talk about the velocity of a
thing without looking outside; the physicists are rather stupid for having
thought otherwise, but it has just dawned on them that this is the case. If
only we philosophers had realized what the problems were that the physicists
had, we could have decided immediately by brainwork that it is impossible to
tell how fast one is moving without looking outside, and we could have made an
enormous contribution to physics." These philosophers are always with us,
struggling in the periphery to try to tell us something, but they never really
understand the subtleties and depths of the problem.

Our inability to detect absolute motion is a result of experiment and not a
result of plain thought, as we can easily illustrate. In the first place,
Newton believed that it was true that one could not tell how fast he is
going if he is moving with uniform velocity in a straight line. In fact,
Newton first stated the principle of relativity as one of his corollaries
to the laws of motion: " The motions of bodies included in a given space are
the same among themselves, whether that space is at rest or moves uniformly
forward in a straight line." Why then did the philosophers not make all this
fuss about "all is relative," or whatever in Newton's time? Because it was not
until Maxwell's theory of electrodynamics was developed that there were
physical laws that suggested that one could measure his velocity without
looking outside; soon it was found experimentally that one could not!

Now, IS it absolutely, definitely, philosophically necessary that one should
not be able to tell how fast he is moving without looking outside? One of the
consequences of relativity was the development of a philosophy which said,
"You can only define what you can measure! Since it is self-evident that one
cannot measure a velocity without seeing what he is measuring it relative to,
therefore it is clear that there is no meaning to absolute velocity. The
physicists should have realized that they can talk only about what they
can measure." But that is the whole problem: whether or not one can define
absolute velocity is the same as the problem of whether or not one can detect
in an experiment, without looking outside, whether he is moving. In other
words, whether or not a thing is measurable is not something to be decided a
priori by thought alone, but something that can be decided only by experiment.
Given the fact that the velocity of light is 300.000 km/s one will find few
philosophers who will calmly state that it is self-evident that if light goes
300.000 km/s in a car, and the car is going 100.000 km/s, that the light
also goes 300.000 km/s past an observer on the ground. That is a shocking
fact to them; the very ones who claim it is obvious find, when you give them
a specific fact, that it is not obvious.

Finally there is even a philosophy which say that one cannot detect ANY motion
except by looking outside. It is simply not true in physics. True, one cannot
perceive a uniform motion in a straight line, but if a whole room were
rotating we would certainly know it, if everybody would be thrown to the wall
--there would be all kinds of "centrifugal" effects. That the earth is turning
on its axis can be determined without looking at the stars, by means of the
so-called Foucault pendulum, for example. Therefore it is not true that "all
is relative"; it is only uniform velocity that cannot be detected without
looking outside. Uniform rotation about a fixed axis CAN be. When this is told
to a philosopher, he is very upset that he did not really understand it,
because to him it seems impossible that one should be able to determine
rotation about an axis without looking outside. If the philosopher is good
enough, after some time he may come back and say, "I understand. We really do
not have such a thing as absolute rotation; we are really rotating relative
to the stars, you see. And so some influence exerted by the stars on the
object must cause the centrifugal force."

Now for all we know, that is true; we have no way, at the present time, of
telling whether there would have been centrifugal force if there were no stars
and nebulae around. We have not been able to do the experiment of removing
all the nebulae and then measuring our rotation, so we simply do not know.
We must admit that the philosopher may be right. He comes back, therefore,
in delight and says, "It is absolutely necessary that the world ultimately
turns out to be this way: absolute rotation means nothing: it is only relative
to the nebulae." Then we say to him, "Now, my friend, is it or is it not obvious
that uniform velocity in a straight line, relative to the nebulae should
produce no effects inside a car?" Now that the motion is no longer absolute,
but is a motion relative to the nebulae, it becomes a mysterious question, and
a question that can be answered only by experiment.

What, then, ARE the philosophical influences of the theory of relativity? If we
limit ourselves to influences in the sense of what kind of new ideas and
suggestions are made to the physicists by the principle of relativity, we could
describe some of them as follows. The first discovery is, essentially, that
even those ideas which have been held for a very long time and which have been
very accurately verified might be wrong. It was a shocking discovery, of
course, that Newton's laws are wrong, after all the years in which they seemed
to be accurate. Of course it is clear, not that the experiments were wrong, but
that they were done over only a limited range of velocities, so small that the
relativistic effects would not have been evident. But nevertheless, we now
have a much more humble point of view of our physical laws -- everything CAN
be wrong!

Secondly, if we have a set of "strange" ideas, such as that time goes slower
when one moves, and so forth, whether we like them or not like them is an
irrelevant question. The only relevant question is whether the ideas are
consistent with what is found experimentally. In other words, the "strange
ideas" need only agree with experiment, ant the only reason that we have to
discuss the behavior of clocks and so forth is to demonstrate that although
the notion of time dilation is strange, it is CONSISTENT with the way we
measure time.

Finally, there is a third suggestion which is a little more technical but which
has turned out to be of enormous utility in our study of other physical laws,
and that is to look at the symmetry of the laws or, more specifically, to look
for the ways in which the laws can be transformed and leave their form the same.
When we discussed the theory of vectors, we noted that the fundamental laws
of motion are not changed when we rotate the coordinate system, and now we
learn that they are not changed when we change the space and time variables
in a particular way, given by the Lorentz transformation. So this idea
of studying the patterns or operations under which the fundamental laws
are not changed has proved to be a very useful one.